(d) GPR profile collected with 180 MHz antenna penetrates only to 3m. The GPR resolves the collapse structure based on lateral changes in reflections and small diffraction hyperbolas at edges. Relatively disturbed reflections and overlapping hyperbolas distinguish the coarse diamictonR1 on the erosional margin. *

Fig. 2. Map of the collapse sinkhole showing the locations of the 25 m long, 3 m deep trenchand geophysical profiles in Fig. 1. Dashed ellipse is the sinkhole edge from trench and geophysics. In the core collected 12 m west of the trench, the top of competent bedrock (white nodular gypsum) is at a depth of 38 m. The water table during the study was at a depth of 14 m.*

Fig. 3. Map of the Valdefierro sinkhole showing a 58 m long, 3 m deep trenchand locations of GPR profiles 1-5 across sinkhole, a cross-section of which is shown in Fig.4. Sinkhole edges at trench and on GPR lines are shown. Open circles depict a river terrace; black circles, the valley bottom. The core collected next to the trench penetrated 10 m of reworked and alluvial sediment. The water table was not reached in the core. A 6-m long, 35-cm wide fissure can be seen on the map aligned with the semi-circular scarp in the terrace (gray).*

Fig. 4. Cross-sections of Valdefierro sinkhole mapped in Fig. 3. (a) Simplified lithostratigraphy in trench across sinkhole consists four deformed fluvial deposits (1-4). Coarse-grained Unit 2 (brown) floors the sinkhole and spans the trench. Overlying units 3 and 4 (orange) are restricted to the core of the synform and so are sinkhole colluvial-fill. Fine-grained, artificial unit R1 (green) spans the trench and contains bricks. Overlying, artificial units R2-R3 pinch out laterally. Units 3 and 4 and also R1-R3 are distinctly bowed down in the synform core indicating that subsidence has been gradual. Normal faults were mapped on the NW limb of the synform (red vertical lines). The faults are attributed to the extension that borders an area of passive bending and shortening. Thus, this is a sagging sinkhole.

(b) GPR profile #1 shows a concave-up prominent reflection (white letters A, B in figure) that defines the limits of the concealed sagging sinkhole. This reflection event geometrically matches and so probably corresponds to the contact between coarse-grained Unit 2 (brown) and the mud-rich unit R1 (green). Another reflection event, D, overlies and truncates against event A-B in its deepest segment. This event geometrically matches and so probably represents the contact between trench units 3-4 and R1. The GPR shows that the sinkhole is significantly larger than estimated from the trench (60 m versus 44 m long). Integration of trench and GPR suggest that the deeper portion of the sinkhole is not represented in the trench and that strata dips in the trench wall are “apparent” and significantly less than true dip. *